1. Technical Field
This disclosure relates generally to camera illumination systems, and more specifically to spatially dynamic illumination systems for camera systems.
2. Description of the Related Art
Active illumination cameras are often used to capture depth information of a scene and to provide an additional sensing dimension to a system. Active illumination camera systems, such as Time of Flight (ToF) or structured light systems, rely on illumination sources to illuminate objects in the field of view of the capturing camera. In order to cover the camera field of view, the illumination source must have a projected field of view at least as large as the field of view of the lens of the capturing camera system. The intensity of illumination on objects in watts per meter squared reduces the farther away the objects are from the camera. The intensity drops off proportional to the inverse square of the distance from the camera. Thus, when illuminating objects far away from the camera, the light source intensity must be higher than when illuminating objects close to the camera to achieve the same signal to noise ratio. This leads to higher power requirements for the illumination source of active illumination systems where objects are farther from the camera. For example, a system that is intended to illuminate objects 2 to 5 meters from the camera may require several watts of illumination power. This higher power requirement for illuminating objects at greater distances makes deployment of active illumination cameras in lower power devices impractical.
FIG. 1 is used to illustrate that the illumination area increases with distance from a camera. The Figure shows the horizontal field of view (HFOV) and the vertical field of view (VFOV) of a camera and illumination source 102 of a device 100. The object 106 distance from the camera and illumination source 102 is labeled as distance D, and the area illuminated by the illumination source is shown as area of illumination at D 104. The area of illumination at D 104 is at least as large as the field of view of the camera at D. The overall number of photons illuminating the object 106 decreases proportionally to the inverse square of the distance D. For example, if object 106 is located at D=one meter and the illumination power falling on the object 106 at D is 90 milliwatts (mW), then if object 106 is moved to D=three meters from the illumination source, the illumination power falling on the object is reduced to 10 mW. Thus, the farther the object 106 from the illumination source, the more illumination power is wasted illuminating areas of the field of view that may not contain meaningful object information. For example, if the desire is to track a person, providing illumination across the entire field of view when the person only fills 10% of the field of view means that 90% of the illumination energy of the illumination source is wasted.